Before I Hang (1940)

Starring Boris Karloff, Evelyn Keyes, Bruce Bennett, Edward Van Sloan, Ben Taggart
Directed by Nick Grinde
(actor & director credits courtesy IMDB.com)

An elderly doctor is imprisoned for euthanizing a patient, but is allowed to work with a prison physician to find a cure for the ravages of age, only to err in using a serum from a killer's blood. 

Karloff stars as the elderly physician in a return to old age makeup similar to what he used in Night Key, and his characterization is excellent, giving a very sympathetic portrayal.  Of course this changes, as does his performance when his blood is contaminated, which for me was not as interesting, and a bit too reminiscent of Jekyll & Hyde, and other films of the era that borrowed that formula, like Black Friday and Invisible Ghost.  Nevertheless I still enjoyed the picture and Karloff's supported by some fine character actors, including Pedro de Cordoba, very good as an aging classical pianist, and Edward Van Sloan as the prison doctor, who had appeared before with Karloff in Frankenstein and The Mummy.  Although about the same length as his other "mad doctor" pictures for Columbia, this one seems a bit more slow-moving and drawn out.